1. Field of the Invention
The present invention relates to a gradation correction curve creation method for creating gradation correction curves used to correct the gradation of images, a gradation correction curve creation apparatus, and a gradation correction curve creation program storage medium containing a gradation correction curve creation program which makes a computer operate as a gradation correction curve creation apparatus.
2. Description of the Related Art
Conventionally, to create a desired image with desired colors, image data of the image is sent to an image output device which will output the image, after performing a color conversion process on the image data according to image output characteristics of the image output device and performing a gradation correction process on the image data to correct its color gradation. In the gradation correction process, a gradation correction curve is defined beforehand in an LUT (lookup table) or the like for each of C (cyan), M (magenta), Y (yellow), and K (black) colors, for example, and image data for the CMYK colors are converted based on the respective gradation correction curves.
Procedures for creating the gradation correction curves involves creating image data which represents a color chart consisting of density patches for each of the C, M, Y, and K colors on a personal computer or the like, sending the image data to the image output device to output the color chart, and measuring the densities of the C, M, Y, and K density patches composing the outputted color chart. By associating the measured densities of the density patches with the values of the image data used to generate the respective density patches, it is possible to create gradation correction curves.
Although gradation correction curves can basically be created as described above, since the gradation correction curves created in this way allow for only the axial directions of the C, M, Y, and K colors, the curves are not adjusted properly in the direction of gray consisting of a mixture of three colors C, M, and Y. Thus, conventional practice involves printing out image data which represent density patches of gray consisting of a mixture of the C, M, and Y colors after making gradation corrections using temporary gradation correction curves created in the manner described above; checking the tint of the density patches of gray visually; fine-adjusting the gradation correction curves; judging whether the gradations of the plain colors C, M, and Y as well as the tint of gray are within tolerances; repeating fine adjustments so that both the gradations and the tint of gray are within the tolerances; and thereby obtaining final gradation correction curves. In this case, fine adjustments are repeated a number of times, expending a great deal of time and effort until finally high-accuracy gradation correction curves are obtained. Besides, considerable experience is required to fine-adjust gradation correction curves properly.
To solve this problem, Japanese Patent Laid-Open No. 2001-245171 discloses a gradation correction curve creation method which determines first gradation correction curves by measuring the densities of density patches of plain colors C, M, and Y, determines second gradation correction curves for each of the C, M, and Y colors by measuring density patches of gray consisting of a mixture of C, M, and Y, internally divides the first gradation correction curves and second gradation correction curves with a predetermined weights assigned, and thereby determines gradation correction curves of the C, M, and Y colors. This method makes it easy to create gradation correction curves without special skills gained by experience.
Regarding the image output device described above, printing machines and ink jet printers as well as electrophotographic printers widely used recently express all colors by superimposing C, M, Y, and K inks or toners. Although three colors C, M, and Y alone should essentially be able to express all colors, the mixture of these three colors cannot provide a sufficiently intense black, and thus K is used to express deep black especially in shadows. K is used widely not only to express deep black, but also to apply a technique known as UCR (Under Color Removal). The UCR is a technique for reducing the total amount of ink or toner by replacing the gray component expressed by three colors C, M, and Y with K. Usually, for example, a color originally expressed by dot area percentage (C, M, Y, K)=(70%, 80%, 90%, 0%) is expressed by dot area percentage (C, M, Y, K)=(0%, 10%, 20%, 70%) as a result of UCR which replaces 70% each of equivalent amounts of the C, M, and Y components of this color with 70% of an equivalent amount of K. Whereas the amount of all the inks added to express this color is 240% in terms of the dot area percentage before the UCR, this amount is reduced to 100% in terms of the dot area percentage after the UCR. In particular, toner is used for electrophotographic printers, and if a number of toners are superimposed at one location, the toners may clog resulting in a printer failure or toners piled up thickly on paper may come off resulting in peeling of films. These problems can be avoided by limiting the total amount of superimposed toners or inks using UCR.
When the total amount of inks is limited using the UCR process, gray consisting of a mixture of C, M, and Y is replaced by K beginning at some point, and thus that part of gradation which has been replaced by K cannot be controlled as gray consisting of a mixture of C, M, and Y. Consequently, the use of gradation correction curves created by the method disclosed in Japanese Patent Laid-Open No. 2001-245171 lowers the reproducibility of colors especially in the shadows where most of the gray component in the color mixture is replaced by K.
In view of the above circumstances, the object of the present invention is to provide a gradation correction curve creation method for creating gradation correction curves which can improve the reproducibility of colors even when a UCR process is performed, a gradation correction curve creation apparatus, and a gradation correction curve creation program storage medium containing a gradation correction curve creation program which makes a computer operate as a gradation correction curve creation apparatus.